Lasers are well known and are usefully employed in many different applications. Despite such familiarity, however, there is still a need for more efficient means by which to generate laser beams. To understand this need, it is helpful to appreciate some of the more basic principles of laser physics.
For purposes of the present invention, two concepts or principles are of particular importance. These are "gain" and "power". Indeed, the main objective of the present invention is to provide means for the efficient use of obtainable gain in a laser medium for the generation of a relatively high power laser beam, i.e. a laser beam in which there is a high concentration of photons. Stated differently, it is desirable to generate a high power laser beam in which there are a large number of photons passing through a given cross-sectional area of the beam per unit of time. To obtain this high concentration, it almost goes without saying that photons must first be generated. This requires a laser medium having a gain which is suffucient to cause the medium to lase.
Simply stated, gain is the probability that the number of photons leaving a medium as output will be more than the number of photons which entered the medium as input. Also, gain is proportional to the number of excited ions per unit volume in the medium. Accordingly, for higher gain, it is desirable there be as many excited state ions as possible in a given gain volume. Further, each medium has threshold gain below which there are no photon emissions from the medium due to the losses which must be overcome before the medium will "lase" and generate a laser beam. Thus, it happens that, for a low gain condition below threshold, there will be excited state ions present in the medium but the medium will not lase. The present invention recognizes that such a supply of excited state ions in a low gain medium can be used to advantage.
As is well known, any laser medium must be "pumped", i.e. the medium must absorb photons and create excited state ions in the medium, before it will lase. One frequently used device for pumping a laser medium is the well known laser diode. The power available from a laser diode, however, is dimensionally dependent and is affected by the arrangement of the individual emitters in the diode. For example, a one half (0.5) watt diode typically consists of ten (10) emitters linearly arranged in a one hundred (100) micron length. Whereas the individual emitters can be placed in a side-by-side relationship with about ten (10) microns distance between emitters, if the individual emitters are to be placed in a stacked relationship, the distance between them must be increased to approximately three hundred (300) microns. Consequently, the power which can be generated by laser diodes has dimensional limitations. Further, laser diodes emit photons in a beam which has large divergence. Therefore, it is difficult to collect sufficient laser diode emitted photons and use them the efficiently "pump" a medium for generation of a relatively high power laser beam. On the other hand, although high power from a small emitter surface area is difficult to attain, it is possible to use the small emitter surface area of a laser diode and focus its output onto a very small volume of a laser medium to obtain a relatively high gain.
Where increase power is the object, laser diodes have some disadvantages. first, as already implied, in order to achieve a high gain condition, they require sophisticated optical arrangements in order to focus their output onto a very small volume of the laser crystal medium. Second, they cannot be effectively grouped to enlarge the gain volume in the laser medium. Thus, although a single one half (0.5) watt diode can be used to create a high gain medium, there is still a relatively low power output and as discussed above, the diodes cannot be effectively "stacked" with other diodes to create a high power laser beam from a high gain medium.
The present invention recognizes that a small volume of a laser medium can be pumped by a laser diode to achieve high gain and generate a laser beam. The present invention further recognizes that when a laser beam is so generated and directed through a large volume of excited state ions in a low gain medium, the result will be a relatively higher power beam. The present invention also recognizes that the low gain medium can be effectively pumped by laser diodes without any connecting optics to create a reservoir of excited state ions in the low gain medium.
In light of the above, it is an object of the present invention to provide a laser diode pumped laser device which will efficiently provide a higher power laser beam. Another object of the present invention is to provide relatively high power laser diode pumped laser device which uses a low gain medium of large volume. Still another object of the present invention is to provide a laser device which efficiently uses a reservoir of excited state ions in a low gain medium to increase the power or a laser beam. Yet another object of the present invention is to provide a diode pumped laser device which is relatively easy to manufacture and comparatively cost effective.